1. Field of the Invention
The invention relates generally to spread spectrum receivers and more particularly to a GPS receiver and a method using memory comparisons to achieve a fast acquisition of a GPS signal.
2. Description of the Prior Art
When first locking to a global positioning system (GPS) signal, a GPS receiver contains uncertainties as to the exact carrier frequency and code phase transmitted from any particular GPS satellite. The carrier frequency uncertainty is due to the unknown difference in frequency between the clock in the GPS receiver and the clock in the particular GPS satellite and the Doppler frequency shift due to their relative motion. The code phase uncertainty is due to the unknown time offset between the GPS receiver and the particular GPS satellite. In order to acquire the GPS signal and achieve lock on the GPS signal, the GPS receiver must resolve both of these uncertainties.
Typical existing GPS receivers perform a trial and error search algorithm in which a code tracking module generates a replica code signal based upon an assumption of the GPS carrier frequency, typically termed a frequency bin, and the GPS code phase; and compares the replica signal to the incoming GPS signal over a period of the GPS code. If the replica and the incoming GPS signal match, the GPS signal is said to have been acquired. If they do not match, the replica code is shifted by one chip and compared again until all possible code phases have been tried. If match has been found after all code phases have been tried, the frequency bin is changed and the code phase shifting and replica comparisons are performed again, and so on until a match is finally found or the GPS receiver concludes that the GPS satellite having that code is not visible. Because each incoming C/A code period is one millisecond long and there are 1023 possible code phases it take about one second to search through all code phases. A typical GPS receiver may require forty frequency bins resulting in a total of about forty seconds to search the GPS signal for a single GPS. There are thirty-two GPS satellites so the total time to acquire and lock to the GPS signal from one GPS satellite may be several minutes. This is considerably longer than is desirable for some GPS applications. For example, when first turned on or when coming out of a tunnel or a parking garage, a GPS receiver having a fast time to first fix begins providing useful information sooner than a slower GPS receiver. Or, a battery operated GPS receiver having a fast time to first fix will operated longer on its battery than a slower GPS receiver. Existing GPS receivers increase the speed of GPS signal acquisition by using multiple replica signals generated in parallel. However, such parallel processing increases expense and power consumption.
Once the GPS receiver acquires and locks to one GPS satellite the number of frequency bins that must be searched to acquire and lock to the GPS signal from additional GPS satellites is reduced because the carrier frequency from the first GPS satellite may now be used as the frequency reference in the GPS receiver. Once the GPS signal from four or more GPS satellites are acquired and locked, a first fix for a GPS-based location and time is derived from information in the GPS signal.
Several workers have developed schemes to improve the time to first fix. For example, Rodal et al. in U.S. Pat. No. 5,594,453 describes a GPS receiver having a rapid acquisition of GPS satellite signals and in U.S. Pat. No. 5,629,708 describes a GPS receiver having an initial adjustment for compensating for drift in reference frequency. Niles in U.S. Pat. No. 5,420,593 describes a method and apparatus for accelerating code correlation searches in initial acquisition and Doppler and code phase in re-acquisition of GPS satellite signals. However, there continues to be a need to improve time to first fix by reducing the time to acquire to the GPS satellite signal.